1. Field of the Invention
This invention relates to internal combustion engines and compressor/expanders, and more particularly to rotary internal combustion engines having thermodynamic cycles differing from the traditional Otto Cycle.
2. Description of the Prior Art
The general functioning of internal combustion rotor rotary engines has typically followed the Otto Cycle, whose thermodynamic pressure-volume relationship is shown in FIG. 1. Typically, as a main rotor lobe passes over an intake port, air is entrained behind it, as shown by the substantially horizontal line at 2. Fuel is injected into the air, and the mixture is compressed 4 into a compression rotor by the front of the lobe. As the main rotor lobe passes over the compression rotor, the mass of air/fuel mixture is transferred from in front of to behind the lobe, and is trapped between the lobe and compression rotor. A spark ignites the mixture at 6, yielding a working pressure on the back of the lobe. As the lobe rotates, the spent gas expands 8, doing work, before being vented to the atmosphere 10. The lobe passes over the intake rotor, leaving the spent gas behind and beginning a new cycle of intake and compression. The spent gas is expelled 12 from the engine by the front of the next main rotor lobe.
The ideal Otto Cycle thermodynamic efficiency (11′) is given by:
                              η          t                =                  1          -                      1                          r              v                              k                -                1                                                                        (        1        )            
Where rv is the cycle's volumetric compression ratio, and where the isentropic value of k for air/fuel mixtures is assumed to be 1.35. For example, the ideal thermal efficiency of an idealized Otto Cycle with a compression ratio of 7.4: I can be found to be 50.4%. The work performed by the cycle can be calculated by integrating the area bounded by the cycle's curves 14. Eq. (1) assumes that the compression and expansion ratios are equivalent. However, asymmetries characteristic of certain rotary engines allow these volumes to differ. In general, when the expansion ratio exceeds the compression ratio the bounded work area is larger as shown in FIG. 2, and the cycle's thermodynamic efficiency is increased—and vice versa. In theory, the maximum work extractable from an engine results from expanding the spent gasses until their pressure is reduced to atmospheric before expelling them 20 (extending the area bounded by the cycle 22). Therefore, asymmetries allowing greater expansion than compression are desirable, and “reverse asymmetries,” which expand less than they compress, are undesirable.
It is also highly desirable for an engine to operate in uniform circular motion because of the mechanical simplicity it affords. A review of the prior art has yielded no engine designs which combine asymmetric compression and expansion and uniform circular motion ill the manner of the present invention. Search of the prior art uncovered designs that would at first appear somewhat similar but which failed to meet the criteria reference hereunder.
When combustion does not occur at the point of maximum compression, the engine must be built, and energy sacrificed, to provide the higher compression pressures and temperatures, but it is then unable to benefit from such pressure ratios. FIG. 3 shows the idealized Otto Cycle compression 30, combustion 32 and expansion 34 strokes for a cycle which over-compresses prior to combustion relative to a cycle where the combustion timing is optimum 38. The engine compresses the gas to a maximum pressure and then reduces the pressure before combusting, resulting in lower peak combustion pressures and temperatures. As can be seen, a significant amount of work 36 is lost.
If spent gas (combustion product) is carried over into the oncoming and compressing charge of fresh air, the thermodynamic efficiency of combustion may be hampered. This is a common characteristic of compression rotors that spin with greater angular velocity than their main rotors. Further, when such compression rotors close-off too soon, prematurely entrapping their pressurized working gases, the gas is stopped from expanding, reducing the work which that volume applies to the engine. If this pressurized gas is introduced into the oncoming charge, its effects on combustion are highly deleterious. To avoid this, some designs have made an allowance for the venting the excess pressure of trapped combustion volumes; but none for removing it's volume before it impairs the following cycle. In these cases, the combustion products become a fraction of the total new gas under compression equal to the volume of the compression chamber divided by the compression chamber volume plus the volume of the fresh air charge. FIG. 4 depicts the pressure-volume relation for such a cycle, where one compression chamber volume is removed from the cycle after half an expansion, the pressure is reduced to atmospheric, and the remaining gas is mixed with the incoming fuel/air charge. In this case, a 7% reduction in work is observed 40.
U.S. Pat. No. 3,498,271 describes a rotary engine with a three-lobed main rotor, partially geared to mesh with a compression chamber and a clearance rotor. Because the compression and clearance rotors are only partially geared, the rotational velocity of these parts is not uniform, accelerating and decelerating as the main rotor lobes interact with them. In addition, the rotational velocity of the two peripheral rotors is three times as great as that of the main rotor. This causes the compression rotor to close off prematurely, entrapping a significant portion of the highly pressurized spent gasses and preventing them from expanding and doing work. The compression rotor also transfers a portion of these gasses back to mix with the incoming charge of fuel/air mixture, reducing the thermal efficiency of combustion. The combustion takes place significantly past the point of maximum compression, further reducing the thermal efficiency resulting from compression. The compression rotor serves as the back wall for the combustion.
The expansion and compression ratios for this engine would be identical if not for this premature sequestration of spent gas.
U.S. Pat. No. 3,990,409 depicts a four-lobed main rotor with uniform rotational velocity one half that of the two peripheral rotors. The rotor is designed to create a high-pressure area upstream of the combustion rotor which will minimize leakage across the seal between the main rotor and the compression rotor. However, in doing this, the engine's compression ratio is greatly reduced, and each compression must work against this higher initial back-pressure, decreasing the net work done by the engine. A non-working volume is incorporated as post-combustion porting to expand a portion of the pressurized gas trapped by the premature closure of the compression chamber; the compression rotor then recycles a one atmosphere rotor volume of spent gasses back into the oncoming charge of fresh and spent gases. The placement of the ignition point on the surface of the main rotor lobes minimizes the rotor travel required before combustion, but still necessitates combustion beyond the point of maximum compression thereby precluding spark advance. The, resulting, shape of the combustion chamber is inefficient having a high ratio of surface area to volume. The compression rotor pressure is vented to the exhaust by means of a separate passage but not purged by fresh incoming air, as such significant non-working gases are reprocessed each cycle.
U.S. Pat. No. 1,136,344 shows a rotary engine with four main rotor lobes and two peripheral rotors geared to have angular velocities four times as great as the main rotor. As in the previous patent, this causes the compression rotor to close off prematurely, trapping a portion of the pressurized combustion products before they are allowed to fully expand. This spent gas is carried back into the oncoming fresh charge of fuel/air mixture, reducing thermodynamic efficiency. Combustion is not initiated until well beyond the point of maximum compression, reducing thermodynamic efficiency, creating negative asymmetry, and requiring the flame front to move too rapidly.
U.S. Pat. No. 2,927,560 illustrates another four-lobed main rotor design with two peripheral rotors partially geared to have twice the angular velocity of the main rotor. This design suffers from very low compression ratios and geometries that allow mixing between compressing and expanding volumes. The design also utilizes complex ducting to move gas volumes, significantly reducing the volumetric efficiency of the engine.
U.S. Pat. No. 892,201 depicts a single-lobe main rotor that passes over lobe valves. The compression lobe valve acts to transfer the compressed gas from the front of the rotor lobe to the back, while the clearance valve acts to separate the spent gas being expelled from the clean intake air. The engine operates with non-uniform motion and very poor internal volumetric efficiency, although it does produce a beneficial compression/expansion asymmetry. Combustion occurs past the point of maximum compression, with the compression lobe valve chamber acting as a partial combustion chamber.
U.S. Pat. Nos. 1,003,263 and 5,595,154 illustrate similar engines with three rotors in uniform circular motion and with peripheral rotors spinning at the same rotational velocity and in the same direction as the main rotor. In both designs, the main rotor compresses gas into an area that acts as a part of the combustion chamber, although combustion does not occur until the main rotor is well past the point of maximum compression. The location of the ignition point, in addition to creating a flame front that must travel in two directions, creates a strong reverse asymmetry. The clearance rotor allows a significant amount of spent gas to be transferred to the intake charge of air, reducing the efficiency of the combustion. Lastly, the oblong geometry of the compression and clearance rotors makes adequate sealing difficult for both designs.
U.S. Pat. No. 1,226,745 shows a rotary engine with a two-lobed main rotor and two peripheral rotors spinning with tile same angular velocities. Although the engine does benefit from asymmetry, combustion is not initiated until well past the point of maximum compression, eliminating much of this benefit. The compression rotor is used as only a part of the combustion chamber, and it carries a volume of pressurized combustion products back into the incoming charge of fresh mixture, reducing the thermodynamic efficiency.
U.S. Pat. No. 1,272,728 depicts a rotary engine with a three-lobed main rotor and non-rotating compression and clearance lobe valves connected to the main rotor via cams. Although a portion of the compression valve chamber is used as a combustion chamber, the combustion is not initiated until after the point of maximum compression. Additionally, the complex combination of valves and cams make ignition timing very difficult for this design.
U.S. Pat. No. 3,297,006 provides for a two lobed rotor and a single compression rotor, rotating at the same angular velocity and pressurized gas fed into the combustion rotor. The compressed mixture is released, via an auxiliary channel where it is combusted, to behind the main rotor lobe. This design includes a significant non-working expansion volume and precludes asymmetry.
U.S. Pat. No. 4,086,880 is a compressor/expander with one main rotor lobe and one compression rotor.